To me it doesn't actually sound like a memory-related issue, 400MB of RAM usage isn't that much for a browser these days, certainly not enough to cause it to hang or unexpectedly close down.
Also, to clarify, when I said other branches of Firefox - they're separate projects run by different developers, it's more like a fork of the Firefox project, I just couldn't think of the right word at the time heh. Working the late shift does that to me sometimes
As for alternative suggestions, I use Chrome at work because we only deploy an old, buggy version of Firefox, I don't use Chrome at home because an add-on I don't want to go without isn't available for it. uBlock Origin is available for Chrome though and that's my second most valued add-on, so maybe give Google Chrome a try, it's regularly updated and I haven't seen many issues with it personally.